This invention relates to a coating composition curable at ambient temperature which contains a very small amount of organic solvent. More particularly, the invention relates to a high solid coating composition curable at ambient temperature which comprises unsaturated vegetable oil modified or unsaturated vegetable oil fatty acid modified alkyd resin having a low viscosity and curing accelerators of metallic alcoholates and/or chelate compounds thereof for the curing of inner portion of coating film. Furthermore, some portion of said alkyd resin of the present invention may be replaced by low molecular weight liquid polybutadiene polymer or modified oils.
The conventional coating compositions which can be dried at ambient temperature generally contain about 40% of organic solvents as thinners which become vapors when they are applied to the surfaces of several substrates and the vapors spread out in the air. Accordingly, the vapors of organic solvents detrimental to health cause public nuisance such as air pollution and they also cause serious losses in view of the saving of resources.
As the measures to solve these problems, there is a method to decrease the content of organic solvents in the coating composition. That is, for example, a coating composition containing much nonvolatile materials may be prepared by using low viscosity resins and nonvolatile low viscosity reactive thinners. However, when the coating composition prepared by this method is applied to form the coating film of, for example, more than 40 microns in dry thickness, shrive rings or wrinklings are caused to occur on the coated surface due to the use of the low molecular weight resins as low viscosity materials and to the small content of organic solvents. That is, since the content of organic solvent is small, the period after coating in which the coated surface comes in contact with the air becomes short, so that the curing in the surface portion of the coating film is faster than that in the inner portion of the coating film, and during the curing step, a considerable difference in the curing rate develops between the surface and the inner portion of the coating film.
Further, much more oxygen is required for the curing of the resin material because it is low in molecular weight, and the surface of the coating film which is in contact with the air is liable to absorb much more oxygen as compared with the inner portion of the coating film, and therefore a hard surface layer is formed in the coating film because the curing of the surface is caused to occur faster than that of inner portion. Accordingly, the diffusion of oxygen to the inner portion of the coating film is delayed and the curing of the inner portion is therefore hindered. For these reasons, surface defects such as wrinkling is formed when the conventional high solid coating composition is used to form the coating film of, for example, more than 40 microns in dry thickness, and what is worse, the inner portion of the coating film remains uncured for a week or more, and therefore the conventional high solid coating composition can not be successfully used. Furthermore, it is difficult to improve the curing of the inner protion of the coating film by using common organic metallic driers so as to be practically accepted.